1. Field of the Invention
This invention relates to an image forming system, and more particularly to an image forming system which can form a plurality of identical images arranged in a row or rows.
2. Description of the Related Art
There has been known an image forming system such as a printer (e.g., a thermal printer, a stencil printer and the like) or a copier which reproduces or outputs an image, for instance, on a printing paper on the basis of an image signal read out from an original, for instance, by a CCD line sensor.
For example, in a stencil printer, an image on an original is read out from the original by an image read-out section, whereby an image signal representing the image is obtained. Then a stencil master material is perforated in an imagewise pattern on the basis of the image signal by an image writing section comprising a thermal head and a platen roller, thereby making a stencil master. The stencil master is wound around a printing drum and ink is transferred through the stencil master to printing papers which are supplied between the printing drum and a press roller pressed against the printing drum. In this manner, the image on the original is printed.
In such a stencil printer, it is sometimes necessary to print an image of an original of a small size (e.g., B6 size) a plurality of times on a larger size printing paper (e.g., of B4 size), for instance, so that four copies of the image are printed on the larger size printing paper side by side in two rows. Such function will be referred to as "contiguous multi-imaging", and the image a plurality of copies of which are to be printed on a printing paper will be referred to as "the image to be multiplied", hereinbelow. When performing contiguous multi-imaging, a line memory such as a RAM has been generally used in order to form a plurality of copies arranged side by side in the direction of the main scanning.
Specifically, as disclosed, for instance, in Japanese Utility Model Publication No. 1(1989)-45170, a plurality of duplicates of image data are made on the line memory by handling the image data as single-bit serial data (binary image data) and storing the same image data at a plurality of addresses by address control of the line memory, and the line memory is thus caused to store image data for contiguous multi-imaging. Accordingly a line memory which is of a single bit in data width is employed.
A thermal head which is employed as an output head in making a stencil master comprises a linear array of a plurality of heater elements each corresponding to one picture element. The heater elements are selectively energized according to image data while the thermal head is being moved relative to a stencil master material in the direction of sub-scanning (the direction substantially perpendicular to the direction in which the linear array of the heater elements extends) to make a stencil master by perforating the stencil master material in an imagewise pattern line by line on the basis of the image data. When the stencil master is made by use of such a thermal head, there has been a problem that heat energy gradually accumulates in each heater element as the stencil master making progresses. This becomes more serious as the stencil master making speed is increased since heat energy generated in the heater element when perforating along a certain line cannot be sufficiently dissipated before starting perforation along the next line. As a result, heat energy accumulates in each heater element according to its heat history and fluctuation in energy condition is generated among the heater elements, which results in deterioration in image quality. When the stencil master making speed is increased by dividing the heater elements of one thermal head into a plurality of blocks which can be driven separately from each other and driving the blocks in parallel, the aforesaid problem is somewhat alleviated. However as the stencil master making speed is further increased, the problem arises again.
There has been proposed "heat-history-based control" in order to overcome the aforesaid problem due to the heat history of each heater element. That is, in the heat-history-based control, heat history of each heater element and those around the heater element is stored in a line memory such as a RAM, and power to be applied to each heater element for perforation of a given line is controlled taking into account the heat history of the heater element and those around the heater element so that the heat energy in the heater elements is uniformed. The heat-history-based control becomes more essential to an image forming system using such a thermal head as the image forming speed increases. See, for instance, Japanese Unexamined Patent Publication Nos. 60(1985)-161163 and 2(1990)-8065.
There has been a demand for a stencil printer which can perform the contiguous multi-imaging at a high speed. In order to meet this demand, the stencil printer must be provided with both the contiguous multi-imaging function and the heat-history-based control function. Such a stencil printer may be realized by separately providing the stencil printer with both a memory for contiguous multi-imaging and a memory for heat-history-based control.
FIG. 13 is a block diagram showing the part for executing contiguous multi-imaging and heat-history-based control of a stencil printer system provided with both a memory for contiguous multi-imaging and a memory for heat-history-based control. In the heat-history-based control of this system, heat-history-based correction image data is made on the basis of the image data for a current line (the line to be formed next) and that for the preceding line and heat-history-based control is performed according to the heat-history-based correction image data. In this system, binary image data in the form of single-bit serial data is input into a data control means 80 for the contiguous multi-imaging. The image data input into the data control means 80 is stored in a RAM 82 at addresses designated by an address control means 84. Normally the address control means 84 increments the address one by one and input image data is stored in the RAM 82 as single-bit data. When a contiguous multi-imaging is on, the image data for the image to be multiplied is stored in a plurality of addresses the number of which is designated by the address control means 84 according to the number of the copies to be formed in the contiguous multi-imaging mode (this number will be referred to as "the number of multiplication", hereinbelow). In this case, though the identical image data is stored at different addresses, the image data is stored at each address as single-bit data.
A data control means 90 for heat-history-based control reads out data in sequence from the RAM 82 and stores the data in a RAM 92 which functions as a two-line memory. At this time, the single-bit data read out from the RAM 82 is divided by the number of blocks (four in this particular example) in the thermal head into four image data fractions which are contiguous in the direction in which the thermal head extends (the direction of the main scanning), and the image data fractions are recorded in the RAM 92 at different bits, whereby the single-bit data read out from the RAM 82 is stored in the RAM 92 as four-bit (equal to the number of blocks in the thermal head) data.
Then a heat-history-based correction image data making section 64 of an output control means 66 reads out the preceding line image data and the current line image data from the RAM 92 and makes heat-history-based correction image data. As shown in FIG. 14, the heat-history-based correction image data is obtained by taking a Boolean intersection of inverted preceding line image data and the current line image data. A data selecting section 67 of the output control means 66 inputs the heat-history-based correction image data made by the heat-history-based correction image data making section 64 into a TPH drive section 72 of a head drive means 70. The TPH drive section 72 drives the blocks 21a to 21d of the thermal head 21 separately from each other on the basis of a control signal from a TPH control signal generating section 74. After the thermal head 21 is driven according to the heat-history-based correction image data, the current line image data is subsequently input into the TPH drive section 72 from the data selecting section 67 and the thermal head 21 is driven according to the current line image data.
That is, when the current line image data for a heater element which was energized by the preceding line image data represents that the heater element is to be energized, the heat-history-based correction image data is set to represent that the heater element is not to be energized, and when the current line data for a heater element which was not energized by the preceding line image data represents that the heater element is to be energized, the heat-history-based correction image data is set to represent that the heater element is to be energized. Accordingly, heater elements which were not energized by the preceding line image data are energized by both the heat-history-based correction image data and the current line image data, whereby they are energized for a longer time, and heater elements which were energized by the preceding line image data are energized by only the current image data, whereby they are energized for a shorter time. That is, in the heat-history-based control in this example, the heat-history-based correction image data and the current line image data are input into the TPH drive section 72 in sequence for each line, and heater elements which were energized by the preceding line image data are energized by only the current line image data while heater elements which were not energized by the preceding line image data are energized by both the heat-history-based correction image data and the current line image data.
However RAMs which are currently available at low cost, especially those having a low capacity suitable for the contiguous multi-imaging, are not of a single-bit structure but of a multiple-bit structure, e.g., four-bit or eight-bit, and single-bit RAMs are comparatively high in cost. When the four-bit or eight-bit RAMs are used as a single-bit RAM, the remaining three or seven bits are held unused in vain, which renders the RAM expensive after all.
Further since using both memories exclusively for the contiguous multi-imaging and for the heat-history-based control is uneconomical and requires a larger space, it is preferred that a single memory be used for both the contiguous multi-imaging and the heat-history-based control. Further when the heater elements of the thermal head are divided into a plurality of blocks in order to increase the imaging forming speed, the memory for the heat-history-based control must be provided with bits of a number not smaller than the number of the blocks, which makes it infeasible to use a memory both for the heat-history-based control and the contiguous multi-imaging. That is, in order to increase the image forming speed while performing the contiguous multi-imaging using a single-bit memory, it is necessary to use a memory for the heat-history-based control separately from the memory for the contiguous multi-imaging.
Further there has been a demand for equalizing action of the system during the contiguous multi-imaging to that during the normal output. Especially in the case where the heater elements of a thermal head are divided into a plurality of blocks, it is not always effective to store identical image data in a memory at a plurality of addresses by address control when the contiguous multi-imaging is to be performed since the data is controlled separately for each block.